The invention relates to a coagulator for an exhaust gas scrubbing system for internal combustion engines, in particular Diesel engines, of the type defined hereinafter.
In such coagulators, which are also known as agglomerators or electrostatic soot filters, the particles, such as soot particles, contained in the exhaust gas are ionized in the electrostatic field that develops between the electrode and the housing, which is polarized as a counterelectrode. As a result, electrical charges adhere to the particles, causing them to coagulate, or in other words mutually attract one another, and combine into larger-sized agglomerates, which are easier to filter out in centrifugal filters (cyclones) located downstream in the exhaust gas scrubbing system. If the field intensity of the electrostatic field between the electrode and the housing is below the ionization field intensity of the particles, a coagulation of the small particles also occurs, because of charge shifts.
Some of the agglomerate thus formed already precipitates out in the coagulator. In time, deposits of electrically conductive agglomerate also form in the insulator that assures the insulated passage of the electrode through the housing, which would short-circuit the insulator and thus cause the coagulator to fail. To avoid this, the surface of the insulator is heated continuously to a temperature of over 400.degree. C. At this temperature, agglomerates are prevented from settling onto the insulator. Any agglomerate deposit that forms after the exhaust gas scrubbing system is shut off can be burned off, or in other words oxidized, by heating the insulator to over 600.degree. C.
In a known coagulator of the type referred to above (German Patent 33 05 601), the cylindrical, stepped insulator has an annular section, protruding at the point in the interior of the coagulator housing at which the electrode emerges from the insulator, on which an incandescent zone is disposed. The incandescent zone is embodied by an electrical resistor wire, which extends in a spiral thick-film heating conductor track over the circumference of the annular section. One disadvantage of this known insulator is its relatively great structural length; another is that it is an additional consumer of current, which requires considerable power, unlike the coagulator itself.